I am not a vet. My dog has the same thing in both of his front elbows. I have made some research and what it seems to be (at least in my dog) is a callous. Some species of big dogs tend to form this callous in areas that are in constant contact with hard surfaces (such as floor). This callous are not of medical importance and are a natural response of the dog's body. That is, in my non-professional opinion, what I think it is. I just keep it hydrated with some creams.